1. Field of the Invention
This invention relates to a film cassette containing one or more negative film units. The cassette is inserted into a camera where the negative film units are exposed and subsequently expelled from the cassette.
2. Description of the Prior Art
Self-developing, peel-apart type film units are well known in the field of instant photography. Each film unit comprises a negative or photosensitive sheet for forming a negative image of a subject, an image-receiving layer for forming a positive subject image and a rupturable pod of processing liquid. A positive image is formed on the positive sheet by means of a well known diffusion transfer process after the pod containing the processing liquid is ruptured and its contents spread between the positive sheet and negative sheet on which a latent image has been formed.
Film units of the aforementioned type are typically exposed in portable, multiple exposure, instant-type photography cameras or processed individually in large format film processing equipment. When employed as a composite (both positive and negative sheets) in a camera, as many as ten film units are provided in a single, light-tight film pack or cassette. The film units are mounted within the cassette in stacked relation with a positive sheet stack on one side and a negative sheet stack on the other side of a pressure plate located within the film cassette. The units are biased in sequence to a proper exposure position by the pressure plate.
The expense and inconvenience of a film cassette holding as few as ten composite film units in a single cassette is a problem. Heretofore, there has been no solution to this problem where instant photography is the goal of a hand-held camera. It has been determined that one way to reduce such expenses and improve convenience would be to limit the film in a film cassette to negative film sheets, only. This arrangement would allow a greater number of negative sheets to be stored within a single cassette over a comparable size cassette containing both positive and negative film sheets. In this regard it should be noted that, with conventional large format film or where on-site development is not necessary, the developing process may be completed outside the camera. This feature provides an opportunity to use the film cassette to contain a large number of negative film sheets and no positive film sheets, for example, 25 to 35 negative sheets in one cassette. The economic benefits of such an arrangement are obvious. After the negative film sheets are exposed in, for example, the large format environment, they are conventionally stored in a lighttight compartment, which may be a transfer cassette, where each exposure or a plurality of exposures are deposited in the transfer cassette and transported to a developing system to be developed by whatever process is appropriate.
In conventional peel-apart, self-developing type film, as is commonly used with certain Polaroid Corporation hand-held camera systems, each positive/negative film unit combination includes a pull-tab that is attached to the leading end thereof. The tab is pulled through a pair of conventional spread rolls by a camera operator, after film exposure, in order to initiate film processing. While effective for the manual processing of self-developing film, such a pull-tab arrangement is not a viable option in, for example, automated film processing apparatus.
Similarly, in conventional integral film of the self-developing type, such as that used with self-developing cameras sold by Polaroid Corporation under their registered trademark Spectra, each positive/negative film unit combination includes a framework between positive and negative film sheets. This two-sheet combination is sufficiently rigid as to allow the exposed film unit to be ejected by a conventional pick member, in the form a hook-like element, that engages the trailing end of the film unit. However, such means for ejection is not an available option when the film unit in the cassette is one of a plurality of thin negative sheets because a negative sheet, by itself, is not rigid enough to be moved longitudinally out of a cassette by the trailing end urging of a conventional pick.
Additionally, single sheets are relatively thin and may be dislodged or prematurely expelled from the light-tight cassette during impact from packaging, shipping, or dropping the cassette as it is unpackaged just prior to insertion into a camera. Maintaining the unexposed integrity of the negative film sheets is critically important to the production of a suitable, visible image on a positive film sheet, following development.
The thinness or flexibility of the negative film sheet creates another problem. The problem is how to obtain the proper orientation of the sheet within the cassette after its exposure so that it may be ejected through a properly located egress opening in the leading end wall of the cassette. Conventional composite structures of the instant developing type do not have this problem because of the relatively rigid frame located between the two sheets. Without the frame, the negative film unit is too flexible and the leading end may droop so as not to be properly oriented with the egress opening. Thus, unique support structure is required in cassettes enclosing unsupported negative film sheets.